Time to Exhaustion

The time to exhaustion is the amount of time that
a fish can swim at a prescribed velocity and is typically estimated from
swim speed tests. Swim tests have generally shown that, when swimming
above sustained speeds, as the swimming velocity increases the amount
of time the velocity can be maintained decreases. Time to exhaustion may
be reported as a constant value derived in the swim test, a default value
selected by FishXing Biologists, or as function of the swim speed equation.

When the relationship between exhaustion times and
swim speeds has not been defined as part of a swim speed study, reasonable
approximations are presumed to be 20 seconds to 200 minutes for prolonged
speeds, and less then 20 seconds for burst speeds (Beamish 1978).

Determining Time to Exhaustion in Swim Speed Tests

In fixed velocity (endurance) tests, the time to
exhaustion is directly measured for each velocity tested. In increasing
velocity (critical) tests velocity is increased in a series of steps at
specific time increments. The assumption required to use this data to
assess fish passage is that the length of the time increment in the critical
velocity test corresponds to the endurance time in a fixed velocity test.
This assumption implies that fish are not fatigued at lower speeds, but
fatigue rapidly as speeds increase (Hunter and Mayor 1986). Brett (1964)
showed that for small fish fatigue curves from fixed and increasing velocity
tests with 60 minute time increments have similar slopes thus supporting
the validity of this assumption. However, fixed velocity tests had slightly
higher endurance times than those predicted from increasing velocity tests.

Selecting Time to Exhaustion in FishXing

When you select a set of swim performance data from
the Literature Swim Speeds Table a time to exhaustion will be automatically
entered into the time to exhaustion field. If the swim speed study reported
time to exhaustion, this data will be entered If
time to exhaustion was not reported in the study, then the default is
arbitrarily set at 10 seconds for burst speeds and 30 minutes for prolonged
speeds. These default values are near the center and the lower end of
the commonly observed time ranges associated with burst and prolonged
speeds, respectively. Since FishXing generally defaults to the middle
to lower end time ranges there is less chance of overestimating the time
to exhaustion.

Constant Swim Speed

When using fixed swimming speeds our recommendation
is to use the reported time. Entering shorter times will result in underestimating
passage capabilities and entering longer times may not be valid because
they are outside the range of those tested. If you enter a time to exhaustion
outside the range used in the original study a warning will be displayed.

Endurance Swim Speed

On the other hand, when using equations with time
as a variable, such as the Hunter and Mayor (1986) equations, it may be
desirable to vary the time to exhaustion. Since
swimming speed and time to exhaustion are related, lowering the time to
exhaustion will increase the swimming speed. It may be desirable to run
the analysis using multiple times to exhaustion within the acceptable
range of values and compare passage performance.